The present invention relates to a semiconductor integrated circuit device and, particularly, relates to a technique that is effectively applied in configuring a preprocessing LSI (hereinafter referred to as “AFE”) for mobile phones equipped with a camera, digital still cameras and digital video cameras.
As related art examined by the present inventors, for example, a technique for configuring AFE is considered, which will be outlined below.
FIG. 11 is a block diagram showing an AFE configuration examined as related art assumed to be improved by the present invention.
As shown in FIG. 11, the AFE examined as related art assumed to be improved by the present invention is comprised of a CDS block, a PGA amplifier, an A/D converter (ADC), a logic circuit (Logic), a charge pump (C.P) and other elements. The CDS block is comprised of a sample and hold amplifier (SH), an offset holding capacity which is provided externally, a CDS amplifier, etc.
Sensor signals are input from an image sensor (CCD) to the CDS block and signal components are extracted therefrom in the CDS block. These signal components are amplified by the PGA amplifier and converted into digital data by the ADC. A sensor signal waveform from the image sensor (CCD) principally comprises a feed-through part (black level) and a signal part (signal component).
As used herein, ADC is an abbreviation of “Analog-to-Digital Converter”, CDS is an abbreviation of “Correlated Double Sampling”, SH is an abbreviation of “Sample Hold”, PGA is an abbreviation of “Programmable Gain Amplifier”, C. P is an abbreviation of “Charge Pump”, and AFE is an abbreviation of “Analog Front End”.
FIG. 12(a) is a circuit diagram showing a specific configuration example of the CDS block shown in FIG. 11. FIG. 12(b) is a diagram showing a configuration example of input capacitors c2, c3 (variable capacitors). FIG. 13 is a waveform chart showing operation timing of the CDS block shown in FIG. 12.
As shown in FIG. 12(a), the CDS block is configured as a switched capacitor type CDS circuit that is comprised of the following elements: input switches T1, T2, T3; input capacitors c0, c2, c3; an offset holding capacitor c1 for holding a signal VR0 to cancel out a sensor offset; feedback capacitors c4, c5; switches T5, T6, T7, T10 for coupling the feedback capacitors c4, c5 to reference voltages VR1, VR2, VR3; switches T8, T9 for coupling the feedback capacitors c4, c5 to a positive output vop and a negative output von; a switch T4 for short circuiting input lines to the input capacitors c2, c3, when signal amplification is performed; and the like. The input capacitors c2, c3 are variable capacitors configured such that their capacitances can be controlled by switching operation of switches as shown in FIG. 12(b). Thereby, the gain of the CDS amplifier can be made variable.
Referring to FIG. 13, this CDS block operates as follows. For each signal that is input from the image sensor (CCD) to a sensor signal input cdsin, its feed-through part f0 is sampled on an input capacitor c0 via an input switch T1. A difference between the feed-through part f0 and a signal VR0 is output at the sample and hold amplifier (SH) which holds a black level signal. The signal VR0 is an offset voltage that is applied to cancel out the sensor offset. Then, the input switch T1 turns off and the output of the sample and hold amplifier (SH) is sampled on an input capacitor c3 via an input switch T3. At the same time, the signal part s0 of each signal input to the sensor signal input cdsin is sampled on an input capacitor c2 via an input switch T2. At this time, feedback capacitors c4, c5 are coupled to reference voltages VR1 to VR3. Then, the input switches T2, T3 and switches T5 to T7 and T10 turn off and switches T4, T8 and T9 turn on, so that the signal is amplified by a gain that is determined by c2/c4 and c3/c5. In this way, the CDS block operates to amplify a difference between the feed-through part (black level) and the signal part of an input signal.
The present applicant investigated into prior art from the viewpoint of “providing a reset circuit that works in case of an excessive input to the CDS amplifier used in the AFE circuit” based on results obtained by the invention. As a result, Patent Documents 1 to 3 are extracted. Patent Document 1 relates to an AFE circuit including a correction logic circuit that corrects offset and eliminates noise. Patent Document 2 relates to an AFE circuit in which overall gain adjustment is distributed between CDS and PGA, thereby achieving a simple circuit structure and reduced chip area of the circuit. Patent Document 3 relates to a CMOS correlated double sampler (CDS) circuit with low noise and low power consumption.
[Patent Document 1]
    Japanese Unexamined Patent Publication No. 2002-57945[Patent Document 2]    Japanese Unexamined Patent Publication No. 2005-45786[Patent Document 3]    Japanese Unexamined Patent Publication No. Hei 11(1999)-103422